This invention is directed to apparatus and a method for determining coating thickness on a substrate, and in particular, it is directed to apparatus and a method for measuring organic and metallic coatings applied to the surface of a steel sheet or strip.
Protective coatings are applied to steel sheet and strip to prevent corrosion. Numerous different surface coatings have been developed to improve both the corrosion resistance and the appearance of steel products. For example metallic coatings, such as zinc and zinc-alloy coatings, are very effective and economical coatings for improving the corrosion resistance of a steel sheet or strip. Such coatings are applied directly to the surface of the steel strip by either electrochemical process or by hot-dipping the strip into a molten zinc bath. Although the primary purpose of metallic coatings is to prevent corrosion of the steel substrate, they can also be used for their decorative features to improve the surface appearance of steel product. For instance, galvanized sheets are often manufactured with a pleasing spangled surface. This produces a product having a bright appearance associated with a high quality steel sheet.
Organic coatings, on the other hand, are primarily used to improve surface appearance. Prepainted steel sheet and strip products are aesthetically pleasing to the eye when they are formed into a finished product. Organic coatings can also improve the durability of steel products. For example, prepainted Galvalume.RTM. sheet used in residential roofing exhibits long-term durability when exposed to our modern outdoor atmosphere that has become increasingly corrosive due to acid rain. Organic coatings are typically in the form of paint systems applied as primers and topcoats directly on top of the metallic coating chemically bonded to the steel substrate. Such organic coatings include enamels, acrylics, epoxies, and the like, and they include different hues to achieve a variety of desired surface appearances.
Steelmakers produce many different prepainted steel sheet products that meet the needs of customer products. To illustrate, prepainted Galvalume consists of 55% aluminum, 43.4% zinc and 1.6% silicon alloy chemically bonded to a steel substrate. An outer paint system comprising polyesters, silicone polyesters, fluorocarbons, plastisols, or the like, is applied directly to the surface of the Galvalume coating. Such prepainted sheets are used in architectural and home product applications for their pleasing appearance. Similarly, a prepainted zinc-nickel coated steel sheet product is manufactured by steel producers for use in fuel tank applications for the automotive industry. In this instance, the finished product comprises a composite material having a zinc-nickel alloy coating bonded to the surface of the steel sheet, and an organic coating comprising an aluminum rich epoxy paint applied as a topcoat.
Because end users of coated steel sheet and strip demand long-term durability in their finished products, it is extremely important to apply protective coatings at specified coating weights to achieve expected product life. Failure to apply a proper coating weight, or coating thickness, can result in premature failure of a customer's finished product due to a breakdown of the barrier protection offered by the metallic coatings and/or the paint systems. Therefore, there has been a long felt need within the coating industry for a device that allows operators to quickly and accurately measure coating thickness on their finished steel sheet and strip products to determine whether the product meets a customer's specification. Such thickness information allows operators to adjust the coating line based on the measurement results.
One such device for measuring coating thickness is disclosed in U.S. Pat. No. 3,340,615 granted to Tooke. Tooke discloses forcing a cutting tool through a film on a substrate to cut a groove into the film. The cut groove has at least one side inclined at a predetermined angle, for example 45.degree., and the film thickness is determined by viewing the 45.degree. inclined surface from above with a microscope. The microscope has a calibrated reticle that converts the 45.degree. slope measurement directly into the length of the sides of a 45.degree. triangle (the film thickness).
Tooke has partially met the long felt need within the industry by providing operators with a simple film-measuring device that, somewhat accurately, allows operators to spot check coating thickness for quality control. However, because present day steel users demand thin gauge steel sheet and strip in their finished products to reduce both weight and cost, modern coating requirements demand even more accurate measurement techniques to spot check coating line quality. Thin gauge steels make it very necessary to insure that proper coating weights are applied to a steel substrate in order to prevent premature product failure due to the breakdown of barrier protection. When a cutting tool is forced through an organic coating as taught by Tooke, the blade motion through the coating causes plastic deformation and elastic springback along the tool path. In the instance of a metallic coating, such as a zinc or a zinc alloy coating, the blade motion causes permanent plastic deformation of the alloy along the cut. Such distortion along the cut prevents accurate measurement of coating thickness and can mislead operators running coating lines.
It has been discovered that grinding, rather than cutting with a tool, eliminates or minimizes elastic springback and the zone of plastic deformation, and therefore, grinding a measuring surface provides a more accurate determination of coating thickness.